Synthetic microspheres and methods of making same

ABSTRACT

A building product incorporating synthetic microspheres having a low alkali metal oxide content is provided. The synthetic microspheres are substantially chemically inert and thus a suitable replacement for natural cenospheres, particularly in caustic environments such as cementitious mixtures. The building product can have a cementitious matrix such as a fiber cement product. The synthetic microspheres can be incorporated as a low density additive and/or a filler for the building product and/or the like.

RELATED APPLICATIONS

[0001] This application claims the benefit of U.S. ProvisionalApplication No. 60/405,790, filed on Aug. 23, 2002, and U.S. ProvisionalApplication No. 60/471,400, filed on May 16, 2003, which are herebyincorporated by reference in their entirety.

BACKGROUND OF THE INVENTION

[0002] 1. Field of the Invention

[0003] Embodiments of this invention generally relate to syntheticmicrospheres and processes for manufacturing the microspheres. Theseembodiments have been developed primarily to provide a cost-effectivealternative to commercially available cenospheres.

[0004] 2. Description of the Related Art

[0005] Any discussion of the prior art throughout the specificationshould in no way be considered as an admission that such prior art iswidely known or forms part of common general knowledge in the field.

[0006] Cenospheres are spherical inorganic hollow microparticles foundin fly ash, which is typically produced as a by-product in coal-firedpower stations. Cenospheres typically make up around 1%-2% of the flyash and can be recovered or “harvested” from fly ash. These harvestedcenospheres are widely available commercially. The composition, form,size, shape and density of cenospheres provide particular benefits inthe formulation and manufacture of many low-density products.

[0007] One of the characterizing features of cenospheres is theirexceptionally high chemical durability. This exceptionally high chemicaldurability is understood to be largely due to the very low content ofalkali metal oxides, particularly sodium oxide, in their composition.Accordingly, low-density composites produced from harvested cenospheresusually have the desirable properties of high strength to weigh ratioand chemical inertness. Chemical inertness is especially important inPortland cement applications, where relative chemical inertness plays animportant role in achieving highly durable cementitious products. Thus,harvested cenospheres have proven to be especially useful in buildingproducts and in general applications where they may come into contactwith corrosive environments where high chemical durability is desirable.

[0008] Despite the known utility of harvested cenospheres, theirwidespread use has been limited to a large extent by their cost andavailability. The recovery of cenospheres in large quantities from flyash is a labor intensive and expensive process. Although it is possibleto increase the recovery of cenospheres from fly ash by modifying thecollection process, the cost of improved recovery does not make thiseconomically viable.

[0009] It may also be possible to alter combustion conditions in powerstations to increase the yield of cenospheres in fly ash. However,combustion conditions in power stations are optimized for coal-burningrather than cenosphere production. It is not economically viable toincrease the yield of cenosphere production at the expense ofcoal-burning efficiency.

[0010] Several methods for producing synthetic microspheres have alsobeen developed and are described in the prior art. Early methods formanufacturing hollow glass microspheres involved combining sodiumsilicate and borax with a suitable foaming agent, drying and crushingthe mixture, adjusting the size of the crushed particles andsubsequently firing the particles. However, these methods suffer fromthe use of expensive starting materials such as borax. Hence, theresulting microspheres are necessarily expensive. In addition, theproduct has poor chemical durability due to the presence of a relativelyhigh percentage of sodium oxide in the resulting glass composition.

[0011] U.S. Pat. No. 3,752,685 describes a method of producing glassmicrospheres from Shirasu, a naturally occurring volcanic rock. Uponheating at 800 to 1000° C. finely divided Shirasu forms hollow glassmicrospheres. However, this method relies on the provision of Shirasu,which is not a widely available starting material.

[0012] U.S. Pat. No. 3,365,315 describes a method of producing glassmicrospheres from glass beads by heating in the presence of water vaporat a temperature of about 1200° C. This method requires the exclusiveuse of pre-formed amorphous glasses as the starting raw materials.

[0013] U.S. Pat. No. 2,978,340 describes a method of forming glassmicrospheres from discrete, solid particles consisting essentially of analkali metal silicate. The microspheres are formed by heating the alkalimetal silicate at a temperature in the range of 1000-2500° F. in thepresence of a gasifying agent, such as urea or Na₂CO₃. Again, thesealkali silicate microspheres suffer from poor chemical durability due toa high percentage of alkali metal oxides.

[0014] U.S. Pat. No. 2,676,892 describes a method of formingmicrospheres from a Macquoketa clay shale by heating particles of theshale to a temperature of 2500-3500° F. The resulting productundesirably has an open pore structure leading to a relatively highwater absorption in an aqueous cementitious environment.

[0015] U.S. Patent Publication No. 2001/0043996 (equivalent ofEP-A-1156021) describes a spray combustion process for forming hollowmicrospheres having a diameter of from 1 to 20 microns. However, thisprocess is unsuitable for making hollow microspheres having a diametersimilar to that of known cenospheres, which is typically about 200microns. In spray combustion processes as described in the reference,rapid steam explosion ruptures larger particles thereby preventingformation of hollow microspheres greater than about 20 microns indiameter.

[0016] Hence, from the foregoing, it will be appreciated that there is aneed for low-cost synthetic microspheres with properties similar tothose of natural microspheres harvested from fly ash. There is also aneed for synthetic microspheres with acceptable chemical durabilitysuitable for incorporation into fiber cement compositions. To this end,there is a particular need for a low-cost, high yield process ofproducing synthetic microspheres from commonly available raw materials.It is an object of the present invention to overcome or ameliorate atleast one of the disadvantages of the prior art, or to provide a usefulalternative.

SUMMARY OF THE INVENTION

[0017] Unless the text clearly requires otherwise, throughout thedescription and the claims, the words “comprise”, “comprising”, and thelike are to be construed in an inclusive sense as opposed to anexclusive or exhaustive sense; that is to say, in the sense of“including, but not limited to”.

[0018] As used herein, the term “synthetic hollow microsphere” or“synthetic microsphere” means a microsphere synthesized as a primarytarget product of a synthetic process. The term does not includeharvested cenospheres which are merely a by-product of burning coal incoal-fired power stations.

[0019] Although the term “microsphere” is used throughout thespecification, it will be appreciated that this term is intended toinclude any substantially spherical microparticle, includingmicroparticles that are not true geometric spheres.

[0020] As used herein, the term “preparing an agglomerate precursor”means a synthetic preparation of an agglomerate precursor by combiningthe various constituents, for example, by a method described below.

[0021] As used herein, the term “primary component” means that thiscomponent is the major constituent of the agglomerate precursor, in thesense that the amount of primary component exceeds the amount of anyother individual constituent.

[0022] In one aspect, the preferred embodiments of the present inventionprovide a building material incorporating a plurality of syntheticmicrospheres having an alkali metal oxide content of less than about 10wt. % based on the weight of the microspheres, wherein the syntheticmicrospheres are substantially chemically inert. In one embodiment, thebuilding material comprises a cementitious matrix. Preferably, thesynthetic microspheres are substantially inert when in contact with thecementitious matrix.

[0023] In preferred embodiments, the synthetic microspheres incorporatedin the building material have an average particle diameter of betweenabout 30 to 1000 microns. In preferred embodiments, the syntheticmicrospheres comprise at least one synthetically formed cavity that issubstantially enclosed by an outer shell. Preferably, the cavitycomprises about 30-95% of the aggregate volume of the microsphere.

[0024] In one embodiment, the building material further comprises one ormore fibers in the cementitious matrix. Preferably, at least one of thefibers is cellulose fibers. Additionally, the building material can alsocomprise a hydraulic binder. In one embodiment, the microspheresincorporated in the building material comprise an aluminosilicatematerial. In another embodiment, the building material further comprisesnatural cenospheres wherein the average particle diameter of the naturalcenospheres is substantially equal to the average particle size of thesynthetic microspheres. The building material can comprise a pillar, aroofing tile, a siding, a wall, or various other types of buildingmaterials.

[0025] From the foregoing, it will be appreciated that certain aspectsof the preferred embodiments provide a building material thatincorporates synthetic microspheres that are substantially chemicallyinert and dimensioned to be used as a substitute for natural harvestedcenospheres. In particular, in certain embodiments, the syntheticmicrospheres can be used as a low density additive and/or fillermaterial for the building material. These synthetic microspheres can beadvantageously incorporated in a cementitious matrix such as a fibercement building product. These and other objects and advantages of thepreferred embodiments of the present invention will become more apparentfrom the following description taken in conjunction with the followingdrawings.

BRIEF DESCRIPTION OF THE DRAWINGS

[0026]FIG. 1 illustrates a phase equilibrium diagram for binary systemNa₂O—SiO₂, the composition being expressed as a weight percentage ofSiO₂;

[0027]FIG. 2 is a schematic illustration of one preferred method ofproducing the agglomerate precursor of one embodiment of the presentinvention;

[0028]FIG. 3 is a scanning electron micrograph of synthetic hollowmicrospheres obtained from Example 1 (Sample 1);

[0029]FIG. 4 is a scanning electron micrograph of synthetic hollowmicrospheres obtained from Example 1 (Sample 2);

[0030]FIG. 5 is a scanning electron micrograph of synthetic hollowmicrospheres obtained from Example 1 (Sample 3);

[0031]FIG. 6 is a scanning electron micrograph of synthetic hollowmicrospheres obtained from Example 2 (Sample 4);

[0032]FIG. 7 is a scanning electron micrograph of synthetic hollowmicrospheres obtained from Example 2 (Sample 5);

[0033]FIG. 8 is a scanning electron micrograph of synthetic hollowmicrospheres obtained from Example 2 (Sample 6);

[0034]FIG. 9 is a scanning electron micrograph of synthetic hollowmicrospheres obtained from Example 3 (Sample 7);

[0035]FIG. 10 is a scanning electron micrograph of synthetic hollowmicrospheres obtained from Example 4 (Sample 12);

[0036]FIG. 11 is a scanning electron micrograph of synthetic hollowmicrospheres obtained from Example 4 (Sample 13);

[0037]FIG. 12 is a scanning electron micrograph of synthetic hollowmicrospheres obtained from Example 5;

[0038]FIG. 13 is a scanning electron micrograph of synthetic hollowmicrospheres obtained from Example 5;

[0039]FIG. 14 is a scanning electron micrograph of synthetic hollowmicrospheres obtained from Example 5;

[0040]FIG. 15 is a scanning electron micrograph of synthetic hollowmicrospheres obtained from Example 6;

[0041]FIG. 16 is a scanning electron micrograph of synthetic hollowmicrospheres obtained from Example 6;

[0042]FIG. 17 is a scanning electron micrograph of synthetic hollowmicrospheres obtained from Example 7.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

[0043] Reference will now be made to the drawings wherein like numeralsrefer to like parts throughout. As described hereinbelow, the preferredembodiments of the present invention provide a chemically durable,synthetic microsphere having properties and characteristics similar tonatural cenospheres harvested from fly ash. The preferred embodimentsalso provide a method for manufacturing the microspheres, including rawmaterial composition and processing, and uses for the microspheres invarious applications, including fiber cement products.

[0044] Synthetic Microspheres

[0045] The synthetic microsphere as described herein generally comprisesa substantially spherical outer wall and a substantially enclosed cavityor void defined by the wall, resembling the general configuration ofharvested cenospheres. However, it will be appreciated that thesynthetic microspheres of certain embodiments can be substantiallysolid. In certain preferred embodiments, the synthetic microsphere hasone or more of the following characteristics, which are also generallycharacteristics of harvested cenospheres:

[0046] an aspect ratio of between about 0.8 and 1;

[0047] (i) a void volume of between about 30 and 95%, based on the totalvolume of the microsphere;

[0048] (ii) a wall thickness of between about 1 to 100 microns and/or 5and 50% of the microsphere radius;

[0049] (iii) a composition comprising about 30 to 85% SiO₂, about 2 to45 wt. %, preferably about 6 to 40 wt. %, A1₂O₃, up to about 30 wt. %divalent metal oxides such as MgO, CaO, SrO, and BaO, about 4 to 10 wt.% monovalent metal oxides such as Na₂O, K₂O, and up to about 20 wt. % ofother metal oxides, including metal oxides which exist in multipleoxidation states such as TiO₂ and Fe₂O₃;

[0050] (iv) a silica to alumina ratio which is greater than about 1;

[0051] (v) an average diameter of between about 40 and 500 microns, morepreferably between about 50 and 300 microns;

[0052] (vi) an outer wall thickness of between about 1 and 50 microns,preferably between about 1 and 30 microns, more preferably between about2.5 and 20 microns;

[0053] (vii) a particle density of between about 0.1 and 20 g/cm³, morepreferably between about 0.2 and 1.5 g/cm³, and more preferably betweenabout 0.4 and 1 g/cm³; or

[0054] (viii) a bulk density of less than about 1.4 g/cm³, preferablyless than about 1 g/cm³.

[0055] In one embodiment, the synthetic microsphere comprises analuminosilicate material having an average particle diameter of betweenabout 30 to 1000 microns and an alkali metal oxide content of less thanabout 10 wt. %, preferably between about 2 to 10 wt. %, based on thetotal weight of the microsphere. In one preferred embodiment, the totalalkali metal oxide content is in the range of about 3 to 9 wt. %, morepreferably about 4 to 8 wt. % based on the total weight of themicrosphere. In some embodiments, the total alkali metal oxide contentof the synthetic microsphere is in the range of about 4 to 6 wt. %,based on the total weight of the microsphere.

[0056] The synthetic microsphere may contain several alkali metaloxides, typically a combination of sodium oxide Na₂O and potassium oxideK₂ 0, which make up the total alkali metal content. The majority of thesodium oxide in the synthetic microspheres is typically derived frombinding agents (e.g. sodium silicate) used in forming the microspheresas will be described in greater detail below. In one embodiment, theamount of sodium oxide in the synthetic microsphere is preferably in therange of about 2 to 10 wt. %, more preferably about 3 to 9 wt. %, morepreferably about 4 to 8 wt. %, and more preferably about 4 to 7 wt. %,based on the total weight of the microsphere. The amount of potassiumoxide in the synthetic hollow microspheres is preferably less than about3 wt. %, more preferably less than about 2 wt. %, and more preferablyless than about 1.5 wt. %, based on the total weight of the microsphere.

[0057] In certain embodiments, the synthetic microsphere furthercomprises one or more chemicals used to form the microspheres. Forexample, the make-up of the wall of the synthetic microsphere mayinclude a binding agent that will be described in greater detail below.Moreover, the synthetic hollow microsphere may also comprise residualamounts of a blowing agent used to form the microsphere as will also bedescribed in greater detail below.

[0058] The synthetic microspheres of the preferred embodiments haveseveral advantages over microspheres known in the prior art. Firstly,the synthetic microspheres comprise an aluminosilicate material.Aluminosilicates are inexpensive and widely available throughout theworld, for example from a large variety of rocks, clays and minerals andalso from waste by-products, particularly bottom ash and fly ash. It isparticularly advantageous that the synthetic microspheres can beprepared from fly ash. Secondly, the presence of only moderatequantities of alkali metal oxide provides the microspheres withacceptably high chemical durability and can be used in the samesituations as known cenospheres. For example, synthetic microspheresaccording to preferred forms of the present invention can withstandhighly caustic environments and harsh autoclaving conditions as typicalof some fiber cement manufacturing processes. By contrast, syntheticmicrospheres produced according to methods known in the prior artgenerally contain high amounts of alkali metal oxides and thus haveunacceptable low chemical durability.

[0059] Furthermore, an average particle diameter of between about 30 and1000 microns for the synthetic microspheres of the preferred embodimentsis advantageous. Particles of this size are known to be relatively safeis building and other materials. When very small particles (e.g. lessthan about 30 microns) are used in building and other materials, therisk of particulates entering the human respiratory system is greatlyincreased. This is highly undesirable since it is known that the entryof particulates into the respiratory system is responsible for manypotentially fatal diseases, which have been well documented. The risk isincreased when composite building materials incorporating the smallparticles are disturbed, for example, by cutting operations. Hence, thelarger average particle diameter of the synthetic microspheres of theembodiments described herein permits the microspheres to be used safelyin a range of applications.

[0060] As will be described in greater detail below, the synthetichollow microsphere of certain preferred embodiments can be formed byfirst preparing an agglomerate precursor, wherein the agglomerateprecursor comprises a primary component, a binding agent, and a blowingagent and then firing the precursor at a predetermined temperatureprofile sufficient to seal the surface of the precursor and activate theblowing agent thereby forming a synthetic hollow microsphere.

[0061] Agglomerate Precursor

[0062] In certain embodiments, the agglomerate precursor is generally asubstantially solid agglomerate mixture comprising a primary component,a binding agent and a blowing agent. Preferably, the amount of primarycomponent comprises about 40 wt. % or more based on the total weight ofthe agglomerate precursor, more preferably about 50 wt. % or more, morepreferably about 70 wt. % or more, more preferably about 80 wt. % ormore, and more preferably about 90 wt. % or more. Preferably, the amountof blowing agent comprises about 0.05 to 10 wt. %, based on the totalweight of the agglomerate precursor, more preferably about 0.1 to 6 wt.%, more preferably about 0.2 to 4 wt. %. The exact amount of blowingagent will depend on the composition of the primary component, the typeof blowing agent and the required density of the final microsphere.

[0063] The preferred ratio of primary component to blowing agent willvary, depending on the composition of each of the ingredients.Typically, the ratio of primary component to blowing agent is in therange of about 1000:1 to 10:1, more preferably about 700:1 to 15:1, andmore preferably about 500:1 to 20:1.

[0064] Preferably, the agglomerate precursor has a water content ofabout 10 wt. % or less, more preferably about 5 wt. % or less, and morepreferably about 3 wt. % or less. The agglomerate precursor issubstantially dry, although a small amount of moisture may be present inthe agglomerate precursor after a solution-based process for forming theprecursor, which is to be described in greater detail below. A smallamount of water may also help to bind particles in the agglomeratetogether, especially in cases where particles in the agglomerateprecursor are water-reactive. In some embodiments, when the agglomerateprecursor has greater than about 10 wt. % water, such as for example 14wt. %., it was found that the agglomerate tend to burst into finesduring the firing process.

[0065] Moreover, the agglomerate precursor preferably has a total alkalimetal oxide content of 10 wt. % or less, and typically in the range ofabout 3 to 10 wt. %, about 4 to 10 wt. % or about 5 to 10 wt. %. A totalalkali metal oxide content of about 10 wt. % or less is advantageous,because microspheres formed from such agglomerate precursors will stillhave acceptably high chemical durability suitable for most applications.

[0066] Preferably, the agglomerate is particulate, having an averageagglomerate particle diameter in the range of about 10 to 1000 microns,more preferably about 30 to 1000 microns, more preferably about 40 to500 microns, and more preferably about 50 to 300 microns.

[0067] Primary Component

[0068] In certain preferred embodiments, the primary component of theagglomerate precursor comprises a low alkali material. The “low alkalimaterial” refers to a material having an alkali metal oxide content ofabout 10 wt. % or less, more preferably about 8 wt. % or less, and morepreferably about 5 wt. % or less. However, in some embodiments, relativehigh alkali materials may still be included in the primary component.The relative high alkali materials may be combined with low alkaliprimary component(s) so that the resulting primary component still has asufficiently low overall alkali metal oxide content. Accordingly, wasteglass powders, such as soda lime glasses (sometimes referred to ascullet) having an alkali content of up to 15 wt. % may be included inthe primary component. However, when combined with other low alkaliprimary component(s), the overall alkali concentration of the primarycomponent should be about 10 wt. % or less.

[0069] Hitherto, it was believed that relatively large amounts of alkalimetal oxides were required to act as a fluxing agent in forming glassmicrospheres from alkali metal silicates (see, for example, U.S. Pat.No. 3,365,315). However, the present inventors have found a method toform synthetic microspheres from commonly available sources of lowalkali content aluminosilicate raw materials without the need for largequantities of additional alkali metal oxides. This will be described ingreater detail below.

[0070] Aluminosilicate materials are well known to the person skilled inthe art. Generally, these are materials having a large component (e.g.,greater than about 50 wt. %, preferably greater than about 60 wt. %) ofsilica (SiO₂) and alumina (Al₂O₃). The amounts of silica and aluminawill vary depending on the source and may even vary within the samesource. Fly ash, for example, will contain varying amounts of silica andalumina depending on the type of coal used and combustion conditions.However, the skilled person will readily understand those materialsclassed as “aluminosilicates”.

[0071] In one embodiment, the primary component of the precursorcomprises at least one aluminosilicate material, preferably about 80 wt.% or more, or about 90 wt. % or more, based on the weight of the primarycomponent. Typically, aluminosilicate materials for use in theembodiments of the present invention have a composition of about 30 to85 wt. % SiO₂; about 2 to 45 wt. % (preferably about 6 to 45 wt. %)Al₂O₃; up to about 30 wt. % (preferably up to about 15 wt. %) divalentmetal oxides (e.g. MgO, CaO, SrO, BaO); up to about 10 wt. % monovalentmetal oxides (e.g. Li₂O, Na₂O, K₂O); and up to about 20 wt. % of othermetal oxides, including metal oxides which exist in multiple oxidationstates (e.g. TiO₂, Fe₂O₃, etc.) Preferably, the mass ratio of silica(SiO₂) to alumina (Al₂O₃) is greater than about 1 in the aluminosilicatematerials used in certain embodiments of the present invention.

[0072] Methods of the present embodiments are not limited to anyparticular source of aluminosilicate material. However, the primarycomponent preferably comprises at least one aluminosilicate materialselected from fly ash (e.g. Type F fly ash, Type C fly ash, etc.),bottom ash, blast-furnace slag, paper ash, basaltic rock, andesiticrock, feldspars, aluminosilicate clays (e.g. kaolinite clay, illiteclay, bedalite clay, betonite clay, china, fire clays, etc.) obsidian,diatomaceous earth, volcanic ash, volcanic rocks, silica sand, silicafume, bauxite, volcanic glasses, geopolymers and combinations thereof.More preferably, the primary component comprises fly ash and/or basalticrock.

[0073] The aluminosilicate material may be either calcined ornon-calcined. The term “calcined” means that the aluminosilicatematerial has been heated in air to a predetermined calcinationtemperature for a predetermined duration so as to either oxidize orpre-react certain component(s) of the aluminosilicate material.Calcination of the aluminosilicate material may be advantageous incertain embodiments of the present invention since the blowing(expansion) process of the microspheres can be sensitive to the redoxstate of multivalent oxide(s) present in the aluminosilicate material.Without wishing to be bound by theory, it is believed that activation ofthe blowing agent is influenced by the release of oxygen from themultivalent oxide(s) present in the aluminosilicate material (e.g., byredox reaction). As an example, a carbonaceous blowing agent may beoxidized to CO₂ by ferric oxide (Fe₂O₃), which is in turn reduced toferrous oxide (FeO). The release of CO₂ from the blowing agent expandsthe microspheres. Hence, by pre-calcining the aluminosilicate materialin air, the relative amount of ferric oxide is increased, which is thenused as a source of oxygen for blowing agents to produce more gas,thereby lowering the density of the microspheres.

[0074] In addition, calcination can promote pre-reaction of oxidecomponents and/or cause partial vitrification in the aluminosilicatematerial, which may be beneficial in the production of high qualitysynthetic microspheres.

[0075] Fly ash is a particularly preferred aluminosilicate primarycomponent due to its low cost and availability. In one preferred form ofthe invention, the primary component comprises about 5 wt. % or more flyash, and more preferably about 10 wt. % fly ash or more, based on thetotal amount of primary component. In another preferred form, theprimary component comprises about 50 wt. % fly ash or more, morepreferably about 70 wt. % fly ash or more, and more preferably about 90wt. % fly ash or more, based on the total amount of primary component.In some embodiments of the present invention, the primary component maybe substantially all fly ash. Fly ash may also be used in the form of afly ash geopolymer, which is formed when fly ash is contacted with anaqueous solution of a metal hydroxide such as sodium hydroxide NaOH orpotassium hydroxide KOH. Fly ash geopolymers are well known in the art.

[0076] In certain embodiments, at least one of the aluminosilicatematerial used preferably comprises an amorphous phase and is eitherpartially or wholly amorphous. In general, a vitrified material issubstantially amorphous.

[0077] In certain embodiments, at least one of the aluminosilicatematerial used preferably has an average primary particle diameter in therange of about 0.01 to 100 microns, more preferably about 0.01 to 100microns, more preferably about 0.05 to 50 microns, more preferably about0.1 to 25 microns, and more preferably about 0.2 to 10 microns.Preferred particle diameters may be achieved by appropriate grinding andclassification. All types of grinding, milling, and overall sizereduction techniques that are used in ceramic industry can be used inembodiments of the present invention. Without limiting to other methodsof size reduction used for brittle solids, preferred methods accordingto embodiments of the present invention are ball milling (wet and dry),high energy centrifugal milling, jet milling, and attrition milling. Ifmore than one aluminosilicate material is to be used, then the multitudeof ingredients can be co-ground together. In one method of the presentinvention, the blowing agent and, optionally the binding agent as willbe described in greater detail below, are added to the aluminosilicatematerial before the milling process. For example all the ingredients canbe co-ground together (e.g. in a wet ball mill), which thensubstantially eliminates the aqueous mixing.

[0078] In an alternative embodiment of the present invention, theprimary component may include waste material(s) and/or otherglass-forming material(s) in addition to the at least onealuminosilicate material. Typical waste materials or other glass-formingmaterial which may be used in this alternative embodiment include wasteglasses (e.g. soda lime glasses, borosilicate glasses or other wasteglasses), waste ceramics, kiln dust, waste fiber cement, concrete,incineration ash, or combinations thereof. The total amount of wastematerial and/or other glass-forming material may be up to about 50 wt. %(e.g. up to about 40 wt. %, up to about 30 wt. %, or up to about 20 wt.%), based on the weight of the primary component. As stated above, it ispreferred that the total amount of alkali metal oxide in the primarycomponent mixture of this type to still be less than about 10 wt. %.

[0079] Blowing Agent

[0080] The blowing agent used in embodiments of the present invention isa substance which, when heated, liberates a blowing gas by one or moreof combustion, evaporation, sublimation, thermal decomposition,gasification or diffusion. The blowing gas may be, for example, CO₂, CO,O₂, H₂O, N₂, N₂O, NO, NO₂, SO₂, SO₃, or mixture Preferably, the blowinggas comprises CO₂ and/or CO.

[0081] Preferably, the blowing agent is selected from powdered coal,carbon black, activated carbon, graphite, carbonaceous polymericorganics, oils, carbohydrates (e.g. sugar, starch, etc.) PVA (polyvinylalcohol), carbonates, carbides (e.g. silicon carbide, aluminum carbide,and boron carbide, etc.), sulfates, sulfides, nitrides (e.g. siliconnitride, boron nitride, aluminum nitride, etc.), nitrates, amines,polyols, glycols, glycerine or combinations thereof. Carbon black,powdered coal, sugar and silicon carbide are particularly preferredblowing agents.

[0082] Preferably, and particularly if the blowing agent is non-watersoluble, the blowing agent has an average particle diameter in the rangeof about 0.01 to 10 microns, more preferably about 0.5 to 8 microns, andmore preferably about 1 to 5 microns.

[0083] Binding Agent

[0084] In preferred embodiment, the agglomerate precursor comprises abinding agent (or binder). The primary function of the binding agent isto bind the particles in the agglomerate together. In some embodiments,the binding agent may act initially to bind particles of the agglomeratetogether during formation of the agglomerate precursor, and then act asa blowing agent during subsequent firing process.

[0085] In general, any chemical substance that is reactive and/oradheres with the aluminosilicate primary component can be used as thebinding agent. The binding agent may be any commercially availablematerial used as a binder in the ceramic industry. Preferably, thebinding agent is selected from alkali metal silicates (e.g. sodiumsilicate), alkali metal aluminosilicate, alkali metal borates (e.g.sodium tetraborate), alkali or alkaline earth metal carbonates, alkalior alkaline earth metal nitrates, alkali or alkaline earth metalnitrites, boric acid, alkali or alkaline earth metal sulfates, alkali oralkaline earth metal phosphates, alkali or alkaline earth metalhydroxides (e.g. NaOH, KOH, or Ca(OH)2), carbohydrates (e.g. sugar,starch, etc.), colloidal silica, inorganic silicate cements, Portlandcement, alumina cement, lime-based cement, phosphate-based cement,organic polymers (e.g. polyacrylates) or combinations thereof. In somecases, fly ash, such as ultrafine, Type C or Type F fly ash, can alsoact as a binding agent.

[0086] The binding agent and blowing agent are typically different fromeach other, although in some cases (e.g. sugar, starch, etc.) the samesubstance may have dual blowing/binding agent properties.

[0087] The term “binder” or “binding agent”, as used herein, includesall binding agents mentioned above, as well as the in situ reactionproducts of these binding agents with other components in theagglomerate. For example, an alkali metal hydroxide (e.g. NaOH) willreact in situ with at least part of the aluminosilicate material toproduce an alkali metal aluminosilicate. Sodium hydroxide may also formsodium carbonate when exposed to ambient air containing CO₂, the rate ofthis process increasing at higher temperatures (e.g. 400° C.). Theresulting sodium carbonate can react with the aluminosilicate materialto form sodium aluminosilicate.

[0088] In certain preferred embodiments, the amount of binding agent isin the range of about 0.1 to 50 wt. % based on the total weight of theagglomerate precursor, more preferably about 0.5 to 40 wt. % and morepreferably about 1 to 30 wt. %.

[0089] It has been unexpectedly found that the properties of the binderor binding agent, and in particular its melting point, affect theproperties of the resulting microspheres. Without wishing to be bound bytheory, it is understood by the present inventors that the binder isresponsible for forming a molten skin around the agglomerate precursorduring or prior to activation of the blowing agent in the firing step aswill be described in greater detail below. Hence, in a preferred form ofthe present invention, the binding agent has a melting point which islower than the melting point of the whole agglomerate precursor.Preferably, the binding agent has a melting point which is less thanabout 1200° C. more preferably less than about 1100° C. and morepreferably less than about 1000° C. (e.g. 700 to 1000° C.).

[0090] It has also been unexpectedly found that the degree ofcrystallinity in the binder phase can have a pronounced effect on theformation kinetics of the molten skin. The degree of crystallinity at agiven temperature may be readily determined from the phase diagram ofoxides present in the mixture. For example, in a simple binary system ofSiO₂ and Na₂O, there are three eutectic points, with the lowest onehaving a liquidus temperature of about 790° C. and a SiO₂ to Na₂O ratioof about 3. As sodium oxide concentration is increased, the liquidustemperature increases sharply, to about 1089° C. at a SiO₂:Na₂O ratio ofabout 1:1. This is illustrated in FIG. 1, which provides a phase diagramof SiO₂—Na₂O. Most other alkali metal oxides behave similarly to sodiumoxide. For example, the K₂O—SiO₂ system has also several eutecticpoints, with the lowest at about 750° C. occurring at a SiO₂ to K₂Oratio of about 2.1. Similarly, Li₂O has several eutectic points with oneat 1028° C. and a ratio of about 4.5.

[0091] In standard glass technology, sodium oxide is known to be astrong fluxing agent. Its addition to silicate glasses lowers themelting point and viscosity of the glass. For example, in a typical sodalime glass composition, there is about 15 wt. % sodium oxide, whichlowers the melting temperature of SiO₂ from about 1700° C. to less thanabout 1400° C. However, in melting commercial glasses, enough time isgiven for the melt to reach the equilibrium concentration throughout theglass mass, normally in the order of hours or longer. Thus, in standardglass technology, sufficient sodium oxide and/or other fluxing agentsare added so that the whole melt has the requisite viscosity-temperaturecharacteristics.

[0092] However, without wishing to be bound by theory, it is understoodby the present inventors that, under the fast reaction kinetics offiring (with a temperature increase as fast as 2000° C. second), one ofthe important requirements for rapid formation of a molten skin aroundthe agglomerate precursor is rapid melting of the binder component.Hence, it is preferred that the binder (present as, for example, sodiumsilicate or sodium aluminosilicate) has a eutectic or near eutecticcomposition. Preferably, the binder is sodium silicate having aSiO₂:Na₂O ratio in the range of about 5:1 to about 1:1, more preferablyabout 4:1 to about 1.5:1, more preferably about 3.5:1 to about 2:1. Itwill be appreciated that other alkali metal oxides (e.g. Li₂O and K₂O)can have the same effect in the binder. However, Na₂O is preferred dueto its low cost.

[0093] It was unexpectedly found that when sodium silicate with an about1:1 ratio of SiO₂:Na₂O was used as binder to formulate the agglomerateprecursor, relatively dense microspheres with a particle density ofabout 1 g/cm³ resulted. However, sodium silicate binder with a SiO₂:Na₂Oratio of about 3:1 resulted in microspheres having a lower density ofabout 0.7 g/cm³. In both cases, the overall concentration of Na₂Orelative to the agglomerate was substantially the same. Under theprinciples of traditional glass-making technology, it would have beenexpected that there would be little or no difference in the finalproducts when using the same amount of fluxing agent. However, thepresent inventors have found that using a eutectic or near eutecticcomposition in the binder, a molten skin is formed rapidly duringfiring, and low density microspheres result, irrespective of the totalamount of fluxing agent in the agglomerate.

[0094] Equally unexpected, it was found that sodium hydroxide showedsubstantially the same trend. Sodium oxide, when used as a binder,reacts with silica present in aluminosilicate powders to form a compoundof sodium silicate. As more sodium hydroxide is added, the ratio ofsilica to sodium oxide is lowered, resulting in binders withprogressively higher melting temperatures.

[0095] Furthermore, the properties of the synthetic microspheres mayalso be dependent on the drying temperature of the agglomerate, and tosome extent, the pressure. For example, a high drying temperature favorsformation of sodium silicate having a lower SiO₂:Na₂O ratio, therebygiving a binder having a higher melting temperature. For example, about5 wt. % of NaOH was found to be an appropriate amount of binder forforming low density microspheres when the agglomerate was dried at about50° C. However, a substantially identical formulation resulted in higherdensity microspheres when the agglomerate was dried at about 400° C. Itwas surprisingly found that, when the agglomerate was dried at about400° C. a lower concentration of NaOH (e.g. about 2-3 wt. %) wasrequired to produce low density microspheres.

[0096] Traditionally, it was believed that a relatively high amount(e.g. 15 wt. %) of sodium oxide was necessary in glass-making technologyto act as a fluxing agent. However, in certain embodiments of thepresent invention, it was surprisingly found that relatively highamounts of sodium oxide are actually less preferred.

[0097] The agglomerate precursor may also include surfactants, whichassist in dispersion of the agglomerate precursor components into anaqueous solution or paste. The surfactants may be anionic, cationic ornon-ionic surfactants.

[0098] As described the above, once the agglomerate precursor is formed,it is fired at a predetermined temperature profile sufficient to sealthe surface of the precursor and activate the blowing agent.

[0099] Methods of Forming the Synthetic Microspheres

[0100] As described above, the synthetic microspheres of certainpreferred embodiments can be formed by first combining the primarycomponent with a binding agent and a blowing agent so as to form anagglomerate precursor in a manner to be described in greater detailbelow. For the formation of substantially solid microspheres, theblowing agent can be left out. The agglomerate precursor is then firedat a pre-determined temperature profile sufficient to activate theblowing agent to release a blowing gas, thereby forming a microspherewith at lease one substantially enclosed void. In embodiments forforming solid synthetic microspheres, the agglomerate precursor is firedat a pre-determined temperature profile that will adequately combine theprimary component with the binding agent.

[0101] In certain preferred embodiments, the temperature profile used inthe firing step substantially fuses the precursor into a melt, reducesthe viscosity of the melt, seals the surface of the precursor andpromotes expansive formation of gas within the melt to form bubbles. Thetemperature profile preferably should maintain the melt at a temperatureand time sufficient to allow gas bubbles to coalesce and form a primaryvoid. After foaming or formation of the primary void, the newly expandedparticles are rapidly cooled, thus forming hollow glassy microspheres.In one embodiment, the temperature profile is preferably provided by afurnace having one or more temperature zones, such as a drop tubefurnace, a vortex type furnace, a fluidized bed furnace or a fuel firedfurnace, with upward or downward draft air streams. A fuel fired furnaceused in certain preferred embodiments of the present invention includesfurnace types in which agglomerated precursors are introduced directlyinto one or a multitude of combustion zones, to cause expansion orblowing of the particles. This is a preferred type of furnace, since theparticles benefit by direct rapid heating to high temperatures, which isdesirable. The heat source may be electric or provided by burning fossilfuels, such as natural gas or fuel oil. One preferred method of heatingis by combustion of natural gas, since this is more economical thanelectric heating and cleaner than burning fuel oil.

[0102] Typically, the peak firing temperature in the firing step is inthe range of about 600 to 2500° C. more preferably about 800 to 2000° C.more preferably about 1000 to 1500° C. and more preferably about 1100 to1400° C. However, it will be appreciated that the requisite temperatureprofile will typically depend on the type of aluminosilicate primarycomponent and blowing agent used. Preferably, the exposure time to thepeak firing temperatures described above will be for a period of about0.05 to 20 seconds, more preferably about 0.1 to 10 seconds.

[0103] Method of Forming Agglomerate Precursor

[0104] As described above, preferred embodiments of the presentinvention also provide methods of preparing an agglomerate precursorthat is suitable for forming a synthetic hollow microsphere therefrom.FIG. 2 provides a schematic illustration of one preferred method 200 offorming the agglomerate precursor.

[0105] As shown in FIG. 2, the method 200 begins with Step 202, whichcomprises providing a primary component of a predetermined size.Preferably, the primary component comprises at least one aluminosilicatematerial. Preferably, the amount of primary component is greater thanabout 40 wt. % based on the total dry weight of the agglomerateprecursor. Preferably, the amount of blowing agent is less than about 10wt. % based on the total dry weight of the agglomerate precursor.Further preferred forms of the primary component and blowing agent aredescribed above.

[0106] As shown in FIG. 2, the method 200 continues with Step 204, whichcomprises mixing the primary component with a blowing agent in water. Incertain preferred embodiments, a binding agent is additionally mixedwith the primary component and the blowing agent in Step 204.Preferably, the amount of binding agent is in the range of about 0.1 to50 wt. %, based on the total dry weight of the agglomerate precursor.Further preferred forms of the binding agent are described above.

[0107] Other additives (e.g. surfactants) may also be added in themixing Step 204, as appropriate. Surfactants may used to assist withmixing, suspending and dispersing the particles. Typically, Step 204provides an aqueous dispersion or paste, which is dried in subsequentsteps. Mixing can be performed by any conventional means, such thatthose used to blend ceramic powders. Examples of preferred mixingtechniques include, but are not limited to, agitated tanks, ball mills,single and twin screw mixers, and attrition mills.

[0108] Subsequent to the mixing process in Step 204, the method 200continues with Step 206, in which the mixture is dried. Drying may beperformed at a temperature in the range of about 30 to 600° C. and mayoccur over a period of up to about 48 hours, depending on the dryingtechnique employed. Any type of dryer customarily used in industry todry slurries and pastes may be used. Drying may be performed in a batchprocess using, for example, a stationary dish or container.Alternatively, drying may be performed in a fluid bed dryer, rotarydryer, rotating tray dryer, spray dryer or flash dryer. Alternatively,drying may also be performed using a microwave oven. It will be readilyappreciated that the optimum drying period will depend on the type ofdrying method employed.

[0109] When drying is performed in a stationary dish or container, it ispreferred that the drying temperature is initially not set too high inorder to avoid water in the mixture boiling violently and thus spewingsolids out of the drying container. In this case, the dryingtemperature, at least initially, is preferably in the range of about 30to 100° C. and more preferably about 40 to 80° C. to avoid initial,rapid boiling of water. However, after initial evaporation of water, thedrying temperature may be increased to temperatures up to about 350° C.which completes the drying process more speedily.

[0110] As shown in FIG. 2, the method 200 of forming the agglomerateprecursor further includes Step 208, which comprises comminuting thedried mixture from Step 206 to form agglomerate precursor particles of apredetermined particle diameter range. However, in some embodiments, thedrying Step 206 and comminuting Step 208 may be performed in a singlestep. Preferably, the dried mixture is comminuted to provide agglomerateprecursor particles having an average particle diameter in the range ofabout 10 to 1000 microns, more preferably about 30 to 1000 microns, morepreferably about 40 to 500 microns, and more preferably about 50 to 300microns. The particle diameter of the agglomerate precursor will affectthe particle diameter of the resultant synthetic hollow microsphere,although the degree of correspondence will, of course, only beapproximate.

[0111] It is preferred that preferred embodiments of the presentinvention provide synthetic hollow microspheres having a controlledparticle diameter distribution. Accordingly, the comminuted agglomerateprecursor may be classified to a predetermined particle diameterdistribution. Alternatively, a controlled particle diameter distributionin the agglomerate precursor may be achieved by the use of spray dryerin the drying Step 206. Spray drying has the additional advantage ofallowing a high throughput of material and fast drying times. Hence, inone preferred embodiment of the present invention, the drying Step 206is performed using a spray dryer. Spray dryers are described in a numberof standard textbooks (e.g. Industrial Drying Equipment, C. M. van'tLand; Handbook of Industrial Drying 2^(nd) Edition, Arun S. Mujumbar)and will be well known to the skilled person. The use of a spray dryerin the preferred embodiments of the present invention has been found tosubstantially eliminate the need for any sizing/classification of theagglomerate precursor.

[0112] Preferably, the aqueous slurry feeding the spray dryer comprisesabout 20 to 90 wt. % solids, more preferably about 25 to 75 wt. %solids, and more preferably about 60 to 70 wt. % solids. In addition tothe agglomerate ingredients described above, the slurry may containfurther processing aids or additives to improve mixing, flowability ordroplet formation in the spray dryer. Suitable additives are well knownin the spray drying art. Examples of such additives are sulphonates,glycol ethers, hydrocarbons, cellulose ethers and the like. These may becontained in the aqueous slurry in an amount ranging from about 0 to 5wt. %.

[0113] In the spray drying process, the aqueous slurry is typicallypumped to an atomizer at a predetermined pressure and temperature toform slurry droplets. The atomizer may be, for example, an atomizerbased on a rotating disc (centrifugal atomization), a pressure nozzle(hydraulic atomization), or a two-fluid pressure nozzle wherein theslurry is mixed with another fluid (pneumatic atomization). The atomizermay also be subjected to cyclic mechanical or sonic pulses. Theatomization may be performed from the top or from the bottom of thedryer chamber. The hot drying gas may be injected into the dryerco-current or counter-current to the direction of the spraying.

[0114] The atomized droplets of slurry are dried in the spray dryer fora predetermined residence time. Typically, the residence time in thespray dryer is in the range of about 0.1 to 10 seconds, with relativelylong residence times of greater than about 2 seconds being generallymore preferred. Preferably, the inlet temperature in the spray dryer isin the range of about 300 to 600° C. and the outlet temperature is inthe range of about 100 to 220° C.

[0115] Use of Synthetic Hollow Microspheres

[0116] The synthetic hollow microspheres according preferred embodimentsof the present invention may be used in a wide variety of applications,for example, in filler applications, modifier applications, containmentapplications or substrate applications. The scope of applications ismuch greater than that of harvested cenospheres due to the low cost andconsistent properties of synthetic microspheres.

[0117] Synthetic microspheres according to the present invention may beused as fillers in composite materials, where they impart properties ofcost reduction, weight reduction, improved processing, performanceenhancement, improved machinability and/or improved workability. Morespecifically, the synthetic microspheres may be used as fillers inpolymers (including thermoset, thermoplastic, and inorganicgeopolymers), inorganic cementitious materials (including materialcomprising Portland cement, lime cement, alumina-based cements, plaster,phosphate-based cements, magnesia-based cements and other hydraulicallysettable binders), concrete systems (including precise concretestructures, tilt up concrete panels, columns, suspended concretestructures etc.), putties (e.g. for void filling and patchingapplications), wood composites (including particleboards, fibreboards,wood/polymer composites and other composite wood structures), clays, andceramics. One particularly preferred use of the microspheres accordingto the present invention is in fiber cement building products. withother materials. By appropriate selection of size and geometry, themicrospheres may be combined with certain materials to provide uniquecharacteristics, such as increased film thickness, improveddistribution, improved flowability etc. Typical modifier applicationsinclude light reflecting applications (e.g. highway markers and signs),industrial explosives, blast energy absorbing structures (e.g. forabsorbing the energy of bombs and explosives), paints and powder coatingapplications, grinding and blasting applications, earth drillingapplications (e.g. cements for oil well drilling), adhesive formulationsand acoustic or thermal insulating applications.

[0118] The synthetic microspheres may also be used to contain and/orstore other materials. Typical containment applications include medicaland medicinal applications (e.g. microcontainers for drugs),micro-containment for radioactive or toxic materials, andmicro-containment for gases and liquids.

[0119] The synthetic microspheres may also be used in to providespecific surface activities in various applications where surfacereactions are used (i.e. substrate applications). Surface activities maybe further improved by subjecting the synthetic microspheres tosecondary treatments, such as metal or ceramic coating, acid leachingetc. Typical substrate applications include ion exchange applications(for removing contaminants from a fluid), catalytic applications (inwhich the surface of the microsphere is treated to serve as a catalystin synthetic, conversion or decomposition reactions), filtration (wherecontaminants are removed from gas or liquid streams), conductive fillersor RF shielding fillers for polymer composites, and medical imaging.

[0120] In one embodiment, the synthetic microspheres of preferredembodiments of the present invention are incorporated in a buildingmaterial. The synthetic microspheres can be incorporated in a compositebuilding material as an additive, low density filler, and/or the like.In one embodiment, the synthetic hollow microspheres are incorporated ina cementitious material. Due in large part to the low alkali metal oxidecontent (e.g. less than 10 wt. %) of the synthetic microspheres, themicrospheres are substantially chemically inert when in contact with thecaustic cementitious material.

[0121] The synthetic microspheres of preferred embodiments can beincorporated in a building material formulation comprising a hydraulicbinder, one or more fibers (e.g. cellulose fibers) For example, thesynthetic microspheres can be incorporated as a low density additive ina fiber cement building material as described in U.S. Pat. No.6,572,697, which is incorporated by reference herein in its entirety.Advantageously, the synthetic microspheres can serve as a substitute forharvested cenospheres in all applications because of the syntheticmicrospheres have substantially the same properties as the cenospheres.

[0122] However, in certain embodiments, the synthetic microspheres canbe manufactured with properties that are superior to that of harvestedcenospheres. For example, in some embodiments, the average aspect ratioof the synthetic microspheres is closer to 1 than the average aspectratio of natural cenospheres, thus providing a microsphere that is morespherical. Moreover, in some embodiments, the average standard deviationof the wall thickness of the synthetic hollow microspheres is less thanthat of cenospheres, which provides a product with a more uniformappearance. These improved properties are achieved through controllingthe processing conditions and raw material in manufacturing themicrospheres.

[0123] The following examples illustrate some preferred methods ofmaking the synthetic hollow microspheres of preferred embodiments of thepresent invention.

EXAMPLE 1

[0124] This example illustrates a method of making syntheticmicrospheres from formulations comprising fly ash, sodium silicate, andsugar.

[0125] Three samples were made by mixing a type F fly ash (ground to anaverage size of about 5.4 microns) with a commercial grade sodiumsilicate solution (SiO₂/Na₂O is about 3.22, about 40% solid content), acommercial grade sugar, and water. The amounts of ingredients are givenin Table 1. The composition of fly ash is given in Table 2. The mixtureswere blended into homogeneous slurry, poured into a flat dish andallowed to solidify at room temperature for about 5 minutes.

[0126] The resulting products were further dried at about 50° C. forabout 20 hours, after which they were ground and sieved to obtainpowders within a size range of about 106 to 180 microns. In the nextstep, for each sample, the powders were fed into a vertical heated tubefurnace at an approximate feed rate of about 0.14 grams/min. The gasflow inside the tube furnace was about 1 litre of air plus 3 litres ofnitrogen per minute. The constant temperature zone of the furnace wasadjusted to provide residence times from less than second toapproximately a few seconds at the peak firing temperatures. The foamedmicrospheres were collected on a funnel shaped collecting device coveredwith a fine mesh screen positioned at the bottom part of the furnace. Amild suction was applied to the end of funnel to aid in collecting themicrospheres. The products were characterized for particle density (e.g.apparent density), percent of water flotation, and approximate particlediameter distribution. The results for various firing temperatures andresidence times are summarized in table 3. FIGS. 3 to 5 show the crosssections of the products. TABLE 1 Sodium silicate Sample No. Fly ashsolution Sugar Water 1 93.1 58.0 3.6 7.0 2 104.8 29.1 3.6 19.2 3 108.021.0 3.6 21.0

[0127] TABLE 2 LOI SiO₂ Al₂O₃ Fe₂O₃ CaO MgO SO₃ Na₂O K₂O TiO₂ Mn₂O₃ P₂O₅Total 0.39 50.63 21.14 7.62 12.39 3.61 0.66 0.63 1.27 1.30 0.17 0.1499.95

[0128] TABLE 3 Residence Apparent Size of Sample Temperature timedensity Water microspheres No. (degree C) (second) (g/cm³) float (%)(micron) 1 1300 0.6-1.1 0.64 81 100-275 1 1300 0.8-1.5 0.78 2 13000.6-1.1 0.87 55 110-240 3 1300 0.6-1.1 1.05  75-225

EXAMPLE 2

[0129] This example illustrates a method of making syntheticmicrospheres from formulations comprising fly ash, sodium silicate, andcarbon black.

[0130] Three samples were made by mixing a type F fly ash (ground to anaverage size of about 5.4 microns) with a commercial grade sodiumsilicate solution (SiO₂/Na₂O is about 3.22, about 40% solid content), acommercial grade carbon black, and water. The amounts of ingredients aregiven in Table 4. The composition of fly ash is given in Table 2. Eachmixture was blended into homogeneous slurry, poured into a flat dish andallowed to solidify at room temperature for about 5 minutes. Theresulting products were further dried at about 50° C. for about 20hours, after which they were ground and sieved to obtain powders withina size range of 106 to 180 microns. In the next step, for each sample,the powders were fed into a vertical heated tube furnace at anapproximate feed rate of about 0.14 grams/min. The gas flow inside thetube furnace is about 1 litre of air plus 3 litres of nitrogen perminute. The constant temperature zone of the furnace was adjusted toprovide residence times from less than a second to approximately a fewseconds at the peak firing temperatures. The foamed microspheres werecollected on a funnel shaped collecting device covered with a fine meshscreen positioned at the bottom part of the furnace. A mild suction wasapplied to the end of funnel to aid in collecting the microspheres. Theproducts were characterized for particle density (e.g. apparentdensity), percent of water floatation, and approximate particle diameterdistribution. The results for various firing temperatures and residencetimes are summarized in Table 5. FIGS. 6 to 8 show the cross sections ofthe products. TABLE 4 Sodium silicate Sample No. Fly ash solution Carbonblack Water 4 95.0 59.0 1.2 7.1 5 100.8 45.0 1.2 18.4 6 106.8 30.0 1.230.1

[0131] TABLE 5 Residence Apparent Size of Sample Temperature timedensity Water microspheres No. (degree C) (second) (g/cm³) float (%)(micron) 4 1300 0.6-1.1 0.87 70 100-275 5 1300 0.6-1.1 0.75 71 100-275 61300 0.6-1.1 0.86 67 110-260

EXAMPLE 3

[0132] This example illustrates a method of making syntheticmicrospheres form formulations comprising fly ash, sodium hydroxide, andcarbon black.

[0133] Three samples were made by mixing a type F fly ash (ground to anaverage size of about 5.4 microns) with a commercial grade solid sodiumhydroxide (flakes), a commercial grade carbon black, and water. Theamounts of ingredients are given in Table 6. The composition of fly ashis given in Table 2. Each mixture was blended into homogeneous slurry,poured into a flat dish and allowed to solidify at room temperature forabout 5 minutes. The resulting products were further dried at about 50°C. for about 20 hours, after which it was ground and sieved to obtainpowders within a size range of about 106 to 180 microns. In the nextstep, the powders were fed into a vertical heated tube furnace at anapproximate feed rate of about 0.14 grams/min. The gas flow inside thetube furnace is about 1 litre of air plus 3 litres of nitrogen perminute. The constant temperature zone of the furnace was adjusted toprovide residence times from less than a second to approximately fewseconds at the peak firing temperatures. The foamed microspheres werecollected on a funnel shaped collecting device covered with a fine meshscreen positioned at the bottom part of the furnace. A mild suction wasapplied to the end of funnel to aid in collecting the microspheres. Theproducts were characterized for particle density (e.g. apparentdensity), percent of water floatation, and approximate particle diameterdistribution. The results are summarized in Table 7. FIG. 9 shows thecross section of the product obtained from Sample 7. TABLE 6 Sample No.Fly ash Sodium hydroxide Carbon black Water 7 112.8 6.0 1.2 39.5 8 116.42.4 1.2 46.6 9 117.6 1.2 1.2 47.0

[0134] TABLE 7 Residence Apparent Size of Sample Temperature timedensity Water microspheres No. (degree C) (second) (g/cm³) float (%)(micron) 7 1300 0.6-1.1 0.65 77 85-290 8 1300 0.6-1.1 0.76 9 13000.6-1.1 0.78 66

EXAMPLE 4

[0135] This example illustrates a method to make synthetic microspheresform formulations consisting of fly ash, basalt, sodium hydroxide, andcarbon black.

[0136] About 94 grams of a type F fly ash and basalt co-ground to anaverage size of about 1 micron were mixed with about 5 grams of solidsodium hydroxide (flakes), about 1 gram of a commercial grade carbonblack, and about 38 ml of water. Several samples were made by changingthe proportions of basalt to fly ash as shown in Table 8. Thecompositions of fly ash and basalt are given in Tables 2 and 9respectively. Each mixture was blended into an homogeneous slurry,poured into a flat dish and allowed to solidify at room temperature forout 5 minutes. The resulting product was further dried at about 50° C.for about 20 hours, after which it was ground and sieved to obtainpowders within a size range of about 106 to 180 microns. In the nextstep, the powders were fed into a vertical heated tube furnace at anapproximate feed rate of about 0.14 grams/min. The gas flow inside thetube furnace is about 1 litre of air plus 3 litres of nitrogen perminute. The constant temperature zone of the furnace was adjusted toprovide residence times from less than a second to approximately fewseconds at the peak firing temperatures. The foamed microspheres werecollected on a funnel shaped collecting device covered with a fine meshscreen positioned at the bottom part of the furnace. A mild suction wasapplied to the end of funnel to aid in collecting the microspheres. Theproducts were characterized for particle density (e.g. apparentdensity), percent of water floatation, and approximate particle diameterdistribution. The results are summarized in Table 10. FIGS. 10 and 11show the cross section of the products of Samples 12 and 13respectively. TABLE 8 Carbon Sample No. Fly ash Basalt Sodium hydroxideblack Water 10 75.2 18.8 5.0 1.0 38.0 11 56.4 37.6 5.0 1.0 38.0 12 37.656.4 5.0 1.0 38.0 13 18.8 75.2 5.0 1.0 38.0

[0137] TABLE 9 LOI SiO₂ Al₂O₃ Fe₂O₃ CaO MgO SO₃ Na₂O K₂O TiO₂ Mn₂O₃ P₂O₅Total 0 46.13 15.81 9.50 9.50 9.60 0 2.78 1.53 2.38 0.25 0.59 98.07

[0138] TABLE 10 Residence Apparent Size of Sample Temperature timedensity Water microspheres No. (degree C) (second) (g/cm³⁾ float (%)(micron) 10 1300 0.8-1.5 0.76 62 11 1300 0.8-1.5 0.77 63 12 1300 0.8-1.50.76 65 100-250 13 1300 0.8-1.5 1.00 44 100-225

EXAMPLE 5

[0139] This example illustrates a method to make synthetic microspheresform a ation comprising basalt, sodium hydroxide, and silicon carbide.

[0140] About 93.5 grams of basalt ground to an average size of about 1micron was mixed with about 5 grams of a commercial grade solid sodiumhydroxide (flakes), about 1.5 grams of a commercial grade siliconcarbide, and about 37.4 ml of water. The composition of basalt is givenin table 9. The mixture was blended into homogeneous slurry, poured intoa flat dish and allowed to solidify at room temperature for about 5minutes. The resulting product was further dried at about 50° C. forabout 20 hours, after which it was ground and sieved to obtain powderswithin a size range of about 106 to 180 microns. In the next step, thepowders were fed into a vertical heated tube furnace at an approximatefeed rate of about 0.14 grams/min. The gas flow inside the tube furnaceis about 1 litre of air plus 3 litres of nitrogen per minute. Theconstant temperature zone of the furnace was adjusted to provideresidence times from less than a second to approximately few seconds atthe peak firing temperatures. The foamed microspheres were collected ona funnel shaped collecting device covered with a fine mesh screenpositioned at the bottom part of the furnace. A mild suction was appliedto the end of funnel to aid in collecting the microspheres. The productswere characterized for particle density (e.g. apparent density), percentof water floatation, and approximate particle diameter distribution. Theresults for various firing temperatures and residence times aresummarized in Table 11. FIGS. 12-14 show the cross section of theproducts. TABLE 11 Apparent Size of Temperature Residence density Waterfloat microspheres (degree C.) time (second) (g/cm³) (%) (micron) 13000.6-1.1 0.61 1250 0.6-1.1 0.56 86 130-260 1200 0.6-1.1 0.59  85-195 11500.6-1.1 1.21 105-240

EXAMPLE 6

[0141] This example illustrates a method to make synthetic microspheresform a formulation comprising fly ash, sodium hydroxide, and siliconcarbide.

[0142] About 93.5 grams of a type F fly ash ground to an average size ofabout 1.3 microms was mixed with about 5 grams of solid sodium hydroxide(flakes), about 1.5 grams commercial grade silicon carbide, and about37.4 ml of water. The composition of the fly ash is given in Table 2.The mixture was blended into homogeneous slurry, poured into a flat dishand allowed to solidify at room temperature for about 5 minutes. Theresulting product was further dried at about 50° C. for about 20 hours,after which it was ground and sieved to obtain powders within a sizerange of about 106 to 180 microns. In the next step, the powders werefed into a vertical heated tube furnace at an approximate feed rate ofabout 0.14 grams/min. The gas flow inside the tube furnace is about 1litre of air plus 3 litres of nitrogen per minute. The constanttemperature zone of the furnace was adjusted to provide residence timesfrom less than a second to approximately few seconds at the peak firingtemperatures. The foamed microspheres were collected on a funnel shapedcollecting device covered with a fine mesh screen positioned at thebottom part of the furnace. A mild suction was applied to the end offunnel to aid in collecting the microspheres. The products werecharacterized for particle density (e.g. apparent density), percent ofwater floatation, and approximate particle diameter distribution. Theresults for various firing temperatures and residence times aresummarized in Table 12. FIGS. 15 and 16 show the cross section of theproducts. TABLE 12 LOI SiO₂ Al₂O₃ Fe₂O₃ CaO MgO SO₃ Na₂O K₂O TiO₂ Mn₂O₃P₂O₅ Total 0.40 61.53 17.91 4.72 7.30 2.91 0.40 2.16 1.39 0.86 0.08 0.2899.94

[0143] TABLE 12 Apparent Size of Temperature Residence density Waterfloat microspheres (degree C.) time (second) (g/cm³) (%) (micron) 14000.6-1.1 0.52 83 1300 0.6-1.1 0.49 96 130-280 1250 0.6-1.1 0.58 105-220

EXAMPLE 7

[0144] This example illustrates a method to make synthetic microspheresform a formulation comprising fly ash, sodium hydroxide, silicon carbideas a primary blowing agent and carbon black a secondary blowing agent.

[0145] About 93.8 grams of a type F fly ash ground to an average size ofabout 1.3 microns was mixed with about 5 grams of solid sodium hydroxide(flakes), about 0.2 grams of a commercial grade silicon carbide, about 1gram of commercial grade carbon black, and about 37.5 ml of water. Thecomposition of the fly ash is given in Table 2. The mixture was blendedinto homogeneous slurry, poured into a flat dish and allowed to solidifyat room temperature for about 5 minutes. The resulting product wasfurther dried at about 50° C. for about 20 hours, after which it wasground and sieved to obtain powders within a size range of about 106 to180 microns. In the next step, the powders were fed into a verticalheated tube furnace at an approximate feed rate of about 0.14 grams/min.The gas flow inside the tube furnace is about 1 litre of air plus 3litres of nitrogen per minute. The constant temperature zone of thefurnace was adjusted to provide residence times from less than a secondto approximately few seconds at the peak firing temperatures. The foamedmicrospheres were collected on a funnel shaped collecting device coveredwith a fine mesh screen positioned at the bottom part of the furnace. Amild suction was applied to the end of funnel to aid in collecting themicrospheres. The products were characterized for particle density (e.g.apparent density), percent of water floatation, and approximate particlediameter distribution. The result is summarized in Table 13. FIG. 17shows the cross section of the product. TABLE 12 Apparent Size ofTemperature Residence density Water float microspheres (degree C.) time(second) (g/cm³) (%) (micron) 1300 0.6-1.1 0.65 82 105-220

EXAMPLE 8

[0146] The compositions (percentage of weight) of synthetic microspheres(“A” and “B”) according to one preferred embodiment of the presentinvention were compared with a sample of commercially availableharvested cenospheres. The results are shown in Table 13. TABLE 13Harvested Synthetic Synthetic Major Oxides Cenosphere Microsphere “A”Microsphere “B” SiO₂ 62.5 58.9 65.8 Al₂O₃ 25.2 17.1 12.8 Fe₂O₃ 3.7 4.53.3 CaO 1.1 7.0 5.2 MgO 1.7 2.8 2.0 Na₂O 1.1 5.2 6.8 K₂O 1.9 1.3 1.0 SO₃0.5 0.4 0.3 Others 2.3 2.8 2.8

EXAMPLE 9

[0147] This example shows typical spray drying conditions used toproduce agglomerate precursors in certain preferred embodiments of thepresent invention.

[0148] Dryer: Bowen Engineering, Inc. No 1 Ceramic Dryer fitted with atwo-fluid nozzle type 59-BS

[0149] Air nozzle pressure: about 20 psi

[0150] Cyclone vacuum: about 4.5

[0151] Inlet/Outlet temperature: about 550° C./120° C.

[0152] Chamber vacuum: about 1.6

[0153] Slurry solids: about 50%

[0154] Agglomerate precursors produced using these spray dryingconditions had a suitable average particle diameter and particlediameter distribution for forming synthetic hollow microspherestherefrom.

[0155] It will be appreciated that embodiments of the present inventionhave been described by way of example only and the modifications ofdetail within the scope of the invention will be readily apparent tothose skilled in the art.

[0156] One preferred method of the present invention advantageouslyprovides a means for producing microspheres in high yield from widelyavailable and inexpensive starting materials, such as fly ash, naturalrocks and minerals. Hence, the method, in its preferred forms, reducesthe overall cost of producing microspheres, and consequently increasesthe scope for their use, especially in the building industry where theuse of presently available cenospheres is relatively limited due totheir prohibitive cost and low availability. Hitherto, it was notbelieved that hollow microspheres could be formed synthetically fromwaste aluminosilicate materials, such as fly ash.

[0157] A further advantage of one embodiment of the present invention,in its preferred form, is that the microspheres produced may betailor-made to suit a particular purpose. For example, the size, densityand composition of the microspheres may be modified, as required, bymodifying the relative amounts of ingredients and/or the temperatureprofile/exposure time during formation.

[0158] Still a further advantage of one embodiment of the presentinvention, in its preferred form, is that the microspheres produced haveacceptably high chemical durability and can withstand, for example, ahighly caustic environment of pH about 12-14 for up to about 48 hours.Thus, microspheres produced according to one preferred embodiment of thepresent invention can withstand aqueous cementitious environments, suchas Portland cement paste.

[0159] Moreover, in most cases, fiber cement products are cured for upto 24 hours in an autoclave that is maintained at temperatures as highas 250° C. Microspheres produced according to one preferred embodimentof the present invention lose minimal amount of mass to dissolution,such as by leaching of silica, retain their shape, and continue to havehigh mechanical strength in fiber cement products, even after exposureto harsh autoclaving conditions.

[0160] Although the foregoing descriptions of certain preferredembodiments of the present invention have shown, described and pointedout some fundamental novel features of the invention, it will beunderstood that various omissions, substitutions, and changes in theform of the detail of the apparatus as illustrated as well as the usesthereof, may be made by those skilled in the art, without departing fromthe spirit of the invention. Consequently, the scope of the presentinvention should not be limited to the foregoing discussions.

What is claimed is:
 1. A building material, comprising: a plurality ofsynthetic microspheres having an alkali metal oxide content of less thanabout 10 wt. % based on the weight of the microspheres, wherein thesynthetic microspheres are substantially chemically inert.
 2. Thebuilding material of claim 1, further comprising a cementitious matrix.3. The building material of claim 2, wherein the synthetic microspheresare substantially chemical inert when in contact with the cementitiousmatrix.
 4. The building material of claim 1, wherein the syntheticmicrospheres have an average particle diameter of between about 30 to1000 microns.
 5. The building material of claim 4, wherein the syntheticmicrospheres comprise at least one synthetically formed cavity that issubstantially enclosed by an outer shell.
 6. The building material ofclaim 5, wherein the at least one cavity comprises about 30-95% of theaggregate volume of the microsphere.
 7. The building material of claim2, further comprising one or more fibers in the cementitious matrix. 8.The building material of claim 7, wherein at least some of the fibersare cellulose fibers.
 9. The building material of claim 1, furthercomprising a hydraulic binder.
 10. The building material of claim 1,wherein the synthetic microspheres comprise an aluminosilicate material.11. The building material of claim 1, further comprising naturalcenospheres wherein the average particle diameter of the naturalcenospheres is substantially equal to the average particle size of thesynthetic microspheres.
 12. The building material of claim 1, whereinthe building material comprises a pillar.
 13. The building material ofclaim 1, wherein the building material comprises a roofing tile
 14. Thebuilding material of claim 1, wherein the building material comprises asiding.
 15. The building material of claim 1, wherein the buildingmaterial comprises a wall.